Image scanning apparatus

1. An image scanning apparatus, comprising: a gray reference member disposed in a conveying path in which an original sheet is to be conveyed, a reflection coefficient of the gray reference member being lower than that of a white reference member; a scanning unit configured to scan an image on the original sheet on a line basis, the scanning unit including a light source configured to illuminate the original sheet when the original sheet passes the gray reference member and a photoelectric conversion element aligned in a scanning direction which is a transverse direction of the conveying path; a light quantity setting unit configured to set a light quantity adjustment value for the light source within a range up to a particular maximum light quantity value; a signal conversion unit configured to convert an analog signal outputted from the photoelectric conversion element to digital data; a correction unit configured to execute shading correction for the digital data based on reference data; a storage unit; and a controller configured to execute a preparatory operation and thereafter execute a scan-preprocessing operation, wherein, as the preparatory operation, the controller is configured to: obtain first black data of one line output from the signal conversion unit in a state where the light source is turned off; obtain white data of one line output from the signal conversion unit when the light source illuminates the white reference member in a state where the light quantity adjustment value is a particular light quantity adjustment value; calculate white and black difference data of one line by subtracting the first black data of one line from the white data of one line at same pixel positions; calculate a white and black difference maximum value which is a maximum value from all pixels of the one line of the white and black difference data and obtain a particular pixel position corresponding to the white and black difference maximum value; and store the white and black difference maximum value and the particular pixel position in the storage unit while associating the white and black difference maximum value with the particular pixel position, wherein, as the scan-preprocessing operation, the controller is configured to: obtain second black data of one line output from the signal conversion unit in a state where the light source is turned off; obtain light gray data of one line output from the signal conversion unit when the light source illuminates the gray reference member at the particular maximum light quantity value; calculate light gray black difference data of one line by subtracting the second black data of one line from the light gray data of one line; calculate a gray white ratio by dividing the white and black difference maximum value at the particular pixel position by the light gray black difference data at the particular pixel position; and calculate the reference data of one line by multiplying the light gray data of one line by the gray white ratio.

2. The image scanning apparatus according to claim 1 , wherein: the white data is formed of a plurality of pieces of color data; and the controller is configured to: calculate the white and black difference maximum value using the white and black difference of particular color data which is one of the plurality of pieces of color data; and obtain, as the particular pixel position, a pixel position of the white and black difference maximum value for each of the plurality of pieces of color data.

3. The image scanning apparatus according to claim 1 , wherein the controller is configured to: obtains, as the white and black difference maximum value, values of a particular number of pixels selected in descending order from the white and black difference data; store the white and black difference maximum value and the particular pixel position for each of the particular number of pixels while associating the white and black difference maximum value with the particular pixel position for each of the particular number of pixels; calculate, as the gray white ratio, an average of values obtained by dividing, for each of the particular number of pixels, the white and black difference maximum value at the particular pixel position by the light gray black difference data at the particular pixel position.

4. The image scanning apparatus according to claim 2 , wherein the white and black difference maximum value of the particular color data is greatest of all of white and black difference maximum values of the plurality of pieces of color data.

5. The image scanning apparatus according to claim 2 , wherein: the plurality of pieces of color data include red color data, blue color data and green color data; and the particular color data is the red color data.

6. An image scanning apparatus, comprising: a gray reference member disposed in a conveying path in which an original sheet is to be conveyed, a reflection coefficient of the gray reference member lower than that of a white reference member; a scanning unit configured to scan an image on the original sheet on a line basis, the scanning unit including a light source configured to illuminate the original sheet when the original sheet passes the gray reference member and a photoelectric conversion element aligned in a scanning direction which is a transverse direction of the conveying path; a light quantity setting unit configured to set a light quantity adjustment value for the light source within a range up to a particular maximum light quantity value; a signal conversion unit configured to convert an analog signal outputted from the photoelectric conversion element to digital data; a correction unit configured to execute shading correction for the digital data based on reference data; a storage unit; and a controller configured to execute a preparatory operation and thereafter execute a scan-preprocessing operation, wherein, as the preparatory operation, the controller is configured to: obtain first black data of one line output from the signal conversion unit in a state where the light source is turned off; obtain white data of one line output from the signal conversion unit when the light source illuminates the white reference member in a state where the light quantity adjustment value is a particular light quantity adjustment value; obtain first gray data of one line output from the signal conversion unit when the light source illuminates the gray reference plate in a state where the light quantity adjustment value is the particular light quantity adjustment value; calculate white and black difference data of one line by subtracting the first black data of one line from the white data of one line at same pixel positions; calculate a white and black difference maximum value which is a maximum value from all pixels of the one line of the white and black difference data and obtain a particular pixel position corresponding to the white and black difference maximum value; and calculate first gray black difference data of one line by subtracting the first black data of one line from the first gray data of one line and calculate a gray black difference maximum value which is the first gray black difference data at the particular pixel position; store the white and black difference maximum value, the gray black difference maximum value and the particular pixel position in the storage unit while associating the white and black difference maximum value, the gray black difference maximum value and the particular pixel position with each other, wherein, as the scan-preprocessing operation, the controller is configured to: obtain second black data of one line output from the signal conversion unit in a state where the light source is turned off; obtain light gray data of one line output from the signal conversion unit when the light source illuminates the gray reference member at the particular maximum light quantity value; obtain second gray data of one line output from the signal conversion unit when the light source illuminates the gray reference plate in a state where the light quantity adjustment value is the particular light quantity adjustment value; calculate light gray black difference data of one line by subtracting the second black data of one line from the light gray data of one line; calculate second gray black difference data of one line by subtracting the second black data of one line from the second gray data of one line; calculate a gray white ratio by dividing the white and black difference maximum value at the particular pixel position by the light gray black difference data at the particular pixel position; calculate a gray ratio by dividing the second gray black difference data at the particular pixel position by the gray black difference maximum value; and calculate the reference data of one line by multiplying together the light gray data of one line, the gray white ratio and the gray ratio.